Data from: Constraints on trait combinations explain climatic drivers of biodiversity: the importance of trait covariance in community assembly

Trade-offs maintain diversity and structure communities along
environmental gradients. Theory indicates that if covariance among
functional traits sets a limit on the number of viable trait combinations
in a given environment, then communities with strong multidimensional
trait constraints should exhibit low species diversity. We tested this
prediction in winter annual plant assemblages along an aridity gradient
using multilevel structural equation modelling. Univariate and
multivariate functional diversity measures were poorly explained by
aridity, and were surprisingly poor predictors of community richness. By
contrast, the covariance between maximum height and seed mass strengthened
along the aridity gradient, and was strongly associated with richness
declines. Community richness had a positive effect on local neighbourhood
richness, indicating that climate effects on trait covariance indirectly
influence diversity at local scales. We present clear empirical evidence
that declines in species richness along gradients of environmental stress
can be due to increasing constraints on multidimensional phenotypes.

权衡机制维持生物多样性，并沿环境梯度塑造群落结构。理论表明，若功能性状（functional traits）间的协变关系限制了特定环境中可行的性状组合数量，那么受到较强多维性状约束的群落，其物种多样性应较低。我们沿干旱梯度，以冬季一年生植物群落为研究对象，采用多层结构方程模型（multilevel structural equation modelling）对该预测进行了验证。干旱程度对单变量及多变量功能多样性指标的解释力较弱，且其作为群落物种丰富度预测因子的表现也出乎意料地不佳。与之相反，最大株高与种子质量间的协变关系随干旱梯度增强，并与物种丰富度下降显著相关。群落物种丰富度对局域邻域丰富度具有正向影响，这表明气候对性状协变关系的作用，会间接影响局域尺度的生物多样性。本研究提供了明确的实证证据，证明沿环境胁迫梯度出现的物种丰富度下降，可能源于多维表型所受约束的不断增强。